1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic isomerization of olefinic hydrocarbons.
2. General Background
Olefinic hydrocarbons are feedstocks for a variety of commercially important addition reactions to yield fuels, polymers, oxygenates and other chemical products. The specific olefin isomer, considering the position of the double bond or the degree of branching of the hydrocarbon, may be important to the efficiency of the chemical reaction or to the properties of the product. The distribution of isomers in a mixture of olefinic hydrocarbons is rarely optimum for a specific application. It is often desirable to isomerize olefins to increase the output of the desired isomer.
Butenes are among the most useful of the olefinic hydrocarbons having more than one isomer. A high-octane gasoline component is produced from a mixture of butenes in many petroleum refineries principally by alkylation with isobutene; 2-butenes (cis- and trans-) generally are the most desirable isomers for this application. Secondary-butyl alcohol and methylethyl ketone, as well as butadiene, are other important derivatives of 2-butenes. Demand for 1-butene has been growing rapidly based on its use as a comonomer for linear low-density polyethylene and as a monomer in polybutene production. Isobutene finds application in such products as methyl methacrylate, polyisobutene and butyl rubber. The most important derivative influencing isobutene demand and butene isomer requirements, however, is methyl t-butyl ether (MTBE) which is experiencing rapid growth in demand as a gasoline component.
Butene isomers rarely are obtained in a refinery or petrochemical product in a ratio matching product demand. In particular, there is a widespread need to increase the proportion of isobutene for MTBE production. Catalytic isomerization to alter the ratio of butene isomers is one solution to this need. Since MTBE is a fuel product and since isomerization competes with increased feedstock processing as a source of butene isomers, an isomerization process must be efficient and relatively inexpensive.
More specifically, a catalytic isomerization process must recognize olefin reactivity. Isobutene in particular readily forms oligomers which could require a reconversion step to yield monomer if produced in excess. The principal problem facing workers in the art therefore is to isomerize olefins to increase the concentration of the desired isomer while minimizing product losses to heavier or lighter products.